MICTURITION

1. What is meant by micturition?

2. Where are the reflex control centers located for micturition?

3. Talk through the events involved in filling of the urinary bladder and micturition.

4. Which division of the autonomics is involved in micturition?

5. Be familiar with the descriptive terms associated with micturition.

During the formation of urine, the tubular fluid flows through the tubules because of a HP difference that exists between Bowman’s capsule and the renal pelvis. The HP in Bowman’s capsule is about 15 to 20 mm Hg and there is almost no HP in the renal pelvis.

Transfer of Urine to the Urinary Bladder

Urine is transported from the renal pelvis to the urinary bladder by peristalsis in the ureters. The ureters enter the urinary bladder at an oblique angle to form a functional valve, the ureterovesicular valve (see Figure 11-5). Once urine has entered the bladder, its backflow into the ureters is prevented as the bladder fills.

Micturition Reflexes

Micturition is the physiologic term for emptying of the bladder. The bladder is allowed to fill before emptying because of reflexes having control centers in the sacral spinal cord and brain stem. Receptors in the bladder wall are stretched during filling and have the reflex ability (activation of sacral spinal cord reflex center) of allowing urine to be evacuated through the neck of the bladder and external sphincter. The brain stem reflex center, however, prevents the contraction of the bladder and relaxation of the external sphincter that would otherwise occur. Normal filling occurs and the cerebral cortex is aroused when the bladder is sufficiently full. Voluntary control intervenes and micturition is permitted when appropriate. Once micturition proceeds, complete emptying is ensured because of another reflex (brain stem) activated by flow receptors in the urethra.

The parasympathetics are the sole motor nerve supply to the detrusor muscle of the bladder. The sympathetics have no effect on micturition, but seem to constrict the neck of the bladder during ejaculation, thus directing the ejaculate through the penile urethra rather than having backflow into the bladder.

Descriptive Terms

Urinary continence is the normal condition of storing urine in the bladder while it fills. Continence is maintained by continuous tone of the external sphincter muscle and by closure of the neck of the bladder, which is augmented by elastic tissue. An incontinent animal dribbles urine at frequent intervals instead of permitting the bladder to fill. Spinal injuries cranial to the sacrum are frequently the cause; in such injuries, the brain stem reflexes do not effectively prevent emptying, and thus emptying is initiated by the sacral reflexes as the bladder fills. Polyuria refers to increased urine output, oliguria means decreased output, and anuria describes the condition of no output. Dysuria is a term used to describe difficult or painful micturition. Stranguria is slow, dropwise, painful discharge of the urine caused by spasm of the urethra and bladder. Stranguria is a clinical sign of feline urologic syndrome, caused by obstruction of the urethra by a plug consisting of struvite (magnesium ammonium phosphate) crystals and mucoid material.

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